Here is a representation of a mass (top) confined to a frictionless surface .. with a hanging mass (bottom) connected by a solid bar. At the moment, you must start the "pendulum" in the straight down position, but you can give either mass whatever velocity you want, and you can change the mass ratio between them. The initial mode is a pure oscillation mode (no net sideways motion) - note that the initial momentum in the system is zero in this initial mode. Try increasing only one of the velocities and see what happens. Try V1=-25, V2=+5, and MR=0.2 !